Bankroll means the money reserved for gambling, and the word is used from both sides of the table.
For a player, it is a budget kept apart from everyday money before play begins. Disciplined players size each bet as a small share of it — often expressed in units — so a run of losses does not end the session early. Approaches such as flat betting or the D’Alembert system are built around a fixed bankroll.
For a casino, the bankroll is the cash reserve available to pay winners. Regulators in many jurisdictions require licensees to hold enough funds to cover player balances, and some require player money to be kept apart from operating funds. For online operators, that means planning liquidity per currency and payment method, and watching exposure on jackpots and high-limit tables.